PROJECT SUMMARY/ABSTRACT Increased incidence and life expectancy has resulted in a growing population of metastatic breast cancer (MBC) patients. MBC patients have poorer quality of life (QOL) and higher rates of functional decline and premature mortality compared to early stage survivors and healthy controls. Interventions are needed to alleviate adverse health effects and allow metastatic patients to function optimally in the years they survive with advanced cancer. However, few health-enhancing interventions exist for women with MBC. Increased physical activity (PA) is consistently associated with fewer treatment-related side effects, higher QOL, increased survival and reduced recurrence and mortality among early stage survivors. Increasing light intensity PA and reducing time spent sedentary may also reduce functional decline, improve QOL and reduced mortality independent of more intense PA. However, there is a paucity of research on PA in MBC patients, and existing interventions have been deemed not feasible due to their focus on intense PA and/or requiring on-site visits. mHealth PA interventions may be particularly useful for these metastatic breast cancer patients because they allow for remote monitoring which facilitates individual tailoring of PA programs to patient's abilities and do not require travel to on-site, supervised PA sessions, reducing participant burden. Yet, no studies have examined a mHealth PA intervention in MBC patients. We propose to address these critical research gaps by testing a highly tailored technology-supported intervention to promote PA of any intensity (i.e. light, moderate or vigorous) via increasing daily steps in MBC patients. This approach may have substantial health benefits for MBC patients, facilitate gradual and safe adoption of more intense PA and be more achievable than high volumes of more strenuous PA. The primary aim of this study is to pilot test the feasibility and acceptability of a 12 week mHealth intervention to increase PA in inactive MBC patients (n=50) using a two-arm randomized control trial (Fit2ThiveMB versus education control). The Fit2ThriveMB intervention consists of a Fitbit, coaching calls and the Fit2ThriveMB smartphone app which provides self-monitoring, a tailored goal-setting tool, real-time tailored feedback, app notifications, and a group message board. We will also examine outcome patterns suggesting the efficacy of Fit2ThriveMB on symptom burden, QOL and functional performance, important outcomes in MBC patients, compared to the education control. Data from this study will provide preliminary effect sizes to be used to assemble an intervention to be evaluated in a larger R01. These data will provide essential evidence to support the feasibility, acceptability and health benefits of increasing PA in MBC patients using a scalable intervention strategy that could be easily integrated into care to improve health and disease outcomes in this population.